The present invention relates to electrochemical detectors for detecting one or more electroactive species and in particular it relates to a voltammetric detector for flowing stream analysis that employs a disposable working electrode.
In recent years there has been increasing demand for continuous monitoring flow-through electrochemical detectors. Of particular interest are voltammetric detectors which measure the faradaic current response at given applied potential, or as the applied potential is varied. Voltammetric detectors have applications which cover many fields and include for example environmental monitoring, process control, and biomedical monitoring. In particular, voltammetric detectors have found application in heavy metal monitoring, clinical chemistry and as detectors for use in high-performance liquid chromatography HPLC).
Voltammetric detectors offer considerable advantages in terms of sensitivity and selectivity over other techniques such as spectroscopy for the analysis of a wide range of chemical species. However, such systems suffer from the fundamental limitation of passivation and/or contamination of the working electrode surface caused by film formation or electrode poisoning caused by products of the electrode reaction; by the adsorption of impurities in the sample or carrier solution; or by the electro-oxidation or electro-reduction of the working electrode surface by the solvent/electrolyte. All these factors usually result in drastic changes in the detector response. The change in response depends on how badly the working electrode is passivated or contaminated. In some cases, the electrode may be rendered totally useless after the analysis of a single sample.
A number of approaches have been taken in obviating electrode passivation and contamination. For example, mechanical scrapers have been employed. These are, however, highly ineffective and can actually damage the electrode surface. They are also noisy and difficult to implement in practical voltammetric detectors. The dropping mercury electrode has been used as a way of presenting a fresh surface for each analysis. Such electrodes are, however, difficult to miniaturise and are unstable in flow systems (resulting in low signal/noise performance). Moreover, they are limited to chemical species that undergo electro-reduction. The use of potential desorption where the working electrode is poised at a potential where the adsorbed species desorb has been described in U.S. Pat. No. 4,059,406. The problem with this approach is that very strongly adsorbed species may not desorb, or the process of desorption may be too slow. Furthermore the electrode potential for desorption must be predetermined. A modification of this method has been described in U.S. Pat. No. 4,556,949 where a continuous voltage pulse train incorporating a cleaning pulse is employed. Again, a problem arises when chemical species are strongly adsorbed or when the electrode itself is oxidised or reduced by the solvent electrolyte. Further, a pulse voltage greatly decreases the signal/noise performance of the detector because of the charging current associated with pulse techniques. Continuous calibration with a standard may be employed to a limited extent to compensate for changing electrode response. However, it is not useful when the signal/noise performance is too low. Also, in general, the analytical precision is lower.
The abovementioned techniques are therefore of limited use in practical situations. In cases of severe passivation or contamination of the working electrode the only practical approach presently is to physically polish the working electrode surface when the electrode response defined by the signal-to-noise ratio is unacceptably low. Polishing is usually done manually with a fine polishing compound such as diamond paste or slurry of alumina powder of 0.05 - 0.01 micron diameter. However, this requires considerable skill on the part of the user so that a uniform and scratch-free surface is obtained.
A solution to these problems that has not been previously realised is to use a fresh electrode every time the signal/noise ratio becomes too low. Hitherto, however, the design of voltammetric detectors for continuous-flow monitoring has been based on sealed or fully enclosed cells where frequent replacement of the working electrode is not practically feasible. By way of example previously designed wall jet detectors such as the one described in U.S. Pat. Nos. 4,059,406 (Fleet), and 4,496,454/4,556,949 (Berger) or thin-layer detectors such as the one proposed by Kissinger (P. T. Kissinger, J. Chem. Educ, 60(1980)308) have provided for a fully enclosed cell where the electrodes are not disposable. Secondly, the working electrode is often fabricated as a permanent part of the cell body as described in U.S. Pat. Nos. 4,496,454 and 4,556,949 (Berger). Thirdly, present methods of fabricating the working electrodes where a solid disk of glassy carbon, gold or platinum is used, make it too costly to use a fresh electrode for each analysis. Further, there is usually considerable variation in performance from one electrode to another.
A considerable number of designs have been proposed for continuous-monitoring voltammetric detectors and the design criteria for these detectors is usually based on the voltammetric technique, measurement mode, electrode geometry, cell geometry, sample delivery and support instrumentation. The wall-jet detector or electrode is particularly suitable as a voltammetric detector because of its high sensitivity, ease of use and hydrodynamic characteristics.
However, there are other important features that sets the wall-jet configuration apart from other detector systems: One is that, by optimising the design, it is feasible to integrate the use of disposable electrodes in a flow-through detector system while maintaining well defined wall-jet hydrodynamics. This capability has not been realised in previous designs of continuous monitoring voltammetric detectors in general and in previous designs of wall-jet detectors in particular. The other feature is that it is possible to provide a reference electrode having a flowing liquid junction in close proximity to the working electrode without disturbing the working electrode response.
According to a paper by Gunasingham and Fleet published in Analytical Chemistry in July 1983 which establishes prior art in this field, when a discrete sample is injected onto the wall-jet electrode, as occurs in HPLC or flow injection analysis (FIA), dispersion of the sample takes place in the hydrodynamic boundary layer and not in the bulk solution as is usually imagined. Attempting to improve the response of wall-jet detector by decreasing cell volume is therefore ineffective and can in some instances decrease sensitivity.
The prior art publication of Gunasingham and Fleet teaches that most previous designs have overlooked fundamental principles of hydrodynamic systems and have not resulted in a structure which satisfies the above requirements. For example, when an electrode is immersed in a flowing stream, three regions of transport activity can be defined: the diffusion layer which is a stationary film adjacent to the electrode of a few hundredths of a millimeter thickness and which is the main region of electrochemical reaction; a hydrodynamic boundary layer where the flow is well defined; and the bulk solution. Because of the unique flow properties of the wall-jet, only electrochemically active species from the jet can actually reach the electrode surface thus excluding species from the bulk solution.
In general, a wall-jet voltammetric detector should satisfy a number of basic criteria in addition to being inexpensive and easy to manufacture. For example, it should be very sensitive to the concentration of electroactive material so that trace amounts can be accurately analysed. Thus, the dimensions of the diffusion layer should be minimised. The hydrodynamic boundary layer in a wall jet should be minimised because the boundary layer is the main region where dispersion of the analyte occurs and by minimising the volume of the boundary layer, band spreading is also minimised. In addition, it is important that the structure of the detector does not interfere with the flow in the boundary layer or else the disturbance in of the boundary layer can greatly affect the detection of the electroactive species and the reliability ability of the results. A further requirement is that the sample volume required to give a steady response should be minimised to permit rapid discrete injections of small volumes without the need for intermittent flushing of the cell thereby avoiding sophisticated low tolerance cell design and permitting automation of on-line voltammetric techniques.
In addition, in a wall-jet detector the free jet made to impinge upon the detector should be obtained under well defined flow conditions and the jet should be stable as defined by the Reynold's number. Furthermore, the counter electrode and reference electrode should be located as close to the working electrode as possible but not so as to interfere with the flow of the wall jet. The electrolysis efficiency should also be optimised to maximise sensitivity in applications such as in HPLC.
There are a number of proposed and known wall-jet voltammetric detectors for use in the application areas described above. However, most efforts concerned with improving existing voltammetric detector designs have focused on cell geometry and location of working electrode and counter electrodes. The general trend has been to minimise the geometric cell volume in order to maximise the ratio of the working electrode area to sample volume. Roston et. al. Analytical Chemistry Vol. 54(1982)1471A and Lunte and Kissinger, Analytical Chemistry Vol 55(1983)1458 have described some thin-layer designs which focus on low cell volume as a criteria for detector performance. A proposed wall-jet cell disclosed in U.S. Pat. No. 4,059,406 also focused on low cell volume as a key criteria for detector performance. These small volume detectors are fully enclosed or sealed devices which preclude regular replacement of the working electrode.
The other practical problem associated with existing wall-jet detectors that are fully enclosed or sealed is that of air bubbles that can get entrapped within the cell. his can lead to increased noise and reduced sensitivity and accuracy.
Because of the unique features offered by the wall-jet configuration, any design should utilise these principles of hydrodynamic operation to the fullest extent to provide an efficient wall-jet voltammetric detector.
An object of the present invention is to provide an improved wall jet detector. The detector provides four features:
i) It provides a wall jet detector which obviates and mitigates the disadvantages associated with the abovementioned existing wall-jet detectors.
ii) It provides a detector where the cell does not have to be sealed or fully enclosed and where the solution level in the detector is defined by the outlet positioning.
iii) it exploits the inherent features provided by (i) and (ii) to enable the use of disposable electrodes thereby obviating the problem of electrode passivation/contamination in routine use without diminishing the hydrodynamic performance of the cell and
iv) through the disposable electrode concept, it is feasible to employ screen printing technology to fabricate chemically coated working electrodes where specific, single layer or multi-layer reagent or membrane coatings are applied to the electrode to make it selective to specific chemical species.
v) it provides a reference electrode which employs a flowing liquid junction thereby eliminating the need for a sealed reference system.
The above features are achieved by providing a wall-jet detector having a large effective cell volume. In addition, however, the cell chamber is not sealed but is actually a well that is opened at the top. The well is partially filled with solution and configured in a way that enables easy placement of the working electrode in the form of a disposable slide. The inlet jet diameter is optimised and the outlet positioned so as to regulate the solution level in the well. The counter and reference electrodes are located in a manner that flow disruption is minimised.
Accordingly, in one aspect of the invention there is provided a wall-jet detector for detecting electrochemical species, said wall-jet detector having a detector housing defining a well which is partially filled with solution, liquid jet producing means coupled to the housing for providing a liquid jet having a jet direction into said detector well and said liquid jet producing means defining a liquid jet inlet to said detector well; a disposable working electrode fabricated on a flat rectangular slide that fits into a slot in the well, said working electrode slide that can be easily positioned in the detector well opposite to said liquid jet inlet at a predetermined distance therefrom, a said working electrode lying in plane substantially perpendicular to the direction of said liquid jet inlet, a reference electrode being coupled to the housing ahd linked to the said well, said detector housing having liquid outlet means for permitting the outflow of solution from said well the arrangement being such that in operative condition, a jet of liquid of a predetermined cross-sectional area is forced into said chamber through said liquid jet inlet means, said working electrode having an electrode cross sectional area in a range of at least five times the cross-sectional area of the liquid jet, said working electrode and said reference electrode being energised for said working electrode to detect electrochemical species in said jet impinging on said working electrode, the detector well being proportioned so that there is no interference with the boundary layer, the stability of the jet being controlled by the linear flow velocity, the inlet jet diameter and the kinematic viscosity selected so that said liquid jet has laminar flow characteristics.
In a preferred embodiment of the invention, a counter electrode is also used. The counter electrode can be fabricated on the same slide as the working electrode or it can be symmetrically placed within the chamber as a separate electrode. The working electrode can be made of a number of materials including carbon, platinum and gold; the counter electrode is made of carbon or platinum; and the reference electrode is usually a silver-silver chloride (Ag/AgCl) electrode. The housing is made of a material depending on the application. For example, Perspex is used for aqueous work and for non aqueous work a material called Kel-F is used. The preferred inlet jet diameter range is 0.1-1.0 mm and the preferred value is 0.3 mm and the preferred distance between the jet nozzle and the working electrode is 1-4 mm and the preferred diameter of the working electrode is selected empirically to give the optimum wall jet cell response but is typically 5 to 10 times the diameter of the inlet jet.